Historically, balloons have been used to study the atmosphere and astronomy. The development of space activities has enabled their field of application to be broadened; today many balloon flights have a technological purpose, for example testing instruments intended to be placed on board satellites.
However, there are two problems to address in general for this kind of balloon:                on the one hand there is a problem of lift, solved by an appropriate envelope volume;        on the other hand there is a problem of propulsion to drive the balloon, which requires recourse to a power source, in general electrical, provided by photovoltaic cells from solar energy.        
Generally, for a guided stratospheric balloon to be autonomous when station-keeping over a duration of several months, it must produce its energy with the aid of photovoltaic cells. Under stratospheric wind conditions, typically greater than 10 m/s, and for a continuous mission, the electrical energy produced during daytime is stored in order to be used at nighttime. The production of daytime electricity rapidly reaches several tens of kilowatts, i.e. several hundreds of m2 of photovoltaic cells, and therefore several hundreds of kilograms. The weight assessment is a major factor for the feasibility of a stratospheric balloon, since the higher the weight to be carried, the greater the size of the balloon, the greater its wind resistance and the more the electrical energy required to power the motors which, when station-keeping, counter the wind to maintain the on-station position of the balloon.
The current solutions lead to covering the upper surface of the balloon with photovoltaic cells, with an unfavourable weight assessment as illustrated in FIG. 1 which schematically shows a balloon of the prior art comprising over a large surface area photovoltaic cells PV capable of picking up solar rays R in order to convert them into electrical energy.
There is notably known from document US 2010/0229850 a solution proposing a balloon equipped with photovoltaic means exhibiting an active face intended to receive solar rays and comprising an envelope comprising a zone transparent to solar rays and a zone including said photovoltaic means, the active face of which is directed toward the inside of said envelope.